The effect of your code is the repeat-y creates a continuous loop along the y axis of you page.

By defining the background position as 200px, this has shifted the start of the loop down 200px, but as your image is the same just repeating this gap is filled with the repeat of your background, so no joy for you.

What I may suggest is something along the lines of a sliding door effect, create an image starting with 200px of white space as suggested by Nathan. Then repeat your image for 1000px save this image.

change the position from 200px to top

Although the background image is now bigger you will get your desired effect.